The present invention relates generally to the production of oxygen and nitrogen from a cryogenic air separation plant, and more particularly to the production of pressurized oxygen using pumped-LOX (liquid oxygen) and the production of at least a portion of nitrogen as pressurized nitrogen.
The most well known cryogenic process for the production of both oxygen and nitrogen is the double-column cycle. This process uses a distillation column system comprising a higher pressure column, a lower pressure column and a reboiler-condenser which thermally links the two columns. Early versions of the double-column cycle produced both nitrogen and oxygen as vapors from the lower pressure column. Recently, it has become commonplace to withdraw the oxygen product from the distillation column system as a liquid, raise the pressure of the liquid oxygen by using either static head or a pump, and warm it in a main heat exchanger by cooling some suitably pressurized stream. This method of oxygen delivery is referred to as pumped-LOX. When large quantities of pressurized nitrogen are also required it is typical to elevate the pressure of the lower pressure column to recover nitrogen at some pressure greater than atmospheric.
Processes of this type are often called elevated pressure, or EP, cycles. Numerous examples of elevated pressure, double column, pumped-LOX cycles exist in the open literature. An example of one such prior art cycle is shown in FIG. 9.
A commercial application for such a process is the production of low purity oxygen (less than 98 mole % oxygen) and nitrogen for Coal Gasification Combined Cycle (CGCC) power and chemical plants. Since an objective of such applications is to produce power, it is essential that the air separation process be energy efficient. The need for high efficiency has given rise to many modifications to the conventional elevated pressure, double-column, pumped-LOX cycle.
One solution for improving the efficiency of the double-column cycle is to utilize a third distillation column as in U.S. Pat. No. 5,682,764 (Agrawal, et al.). This patent teaches the use of a third column which operates at a pressure intermediate that of the higher and lower pressure columns. This third column receives a vapor air feed which is at a lower pressure than the main air feed to the higher pressure column. This intermediate pressure column has a condenser but no reboiler, and produces liquid nitrogen reflux for the lower pressure column. Power consumption is reduced by only having to compress a fraction of the feed air to the pressure of the higher pressure column.
Another patent which teaches the use of a third column to improve efficiency is U.S. Pat. No. 5,678,426 (Agrawal, et al.). This patent also teaches the use of a third column which operates at a pressure intermediate that of the higher and lower pressure columns. This third column receives oxygen-enriched liquid from the bottom of the higher pressure column as a feed. This intermediate pressure column contains both a reboiler and a condenser, and produces a nitrogen-rich stream from its top and a further-oxygen-enriched liquid from its bottom.
Another patent which teaches the use of a third column to improve efficiency is disclosed in U.S. Pat. No. 4,254,629 (Olszewski). Olszewski teaches the use of a third intermediate pressure column which functions much like that of U.S. Pat. No. 5,682,764. Olszewski also discloses a four-column version which has a pair of double columns in parallel. As taught by Olszewski, both lower pressure columns operate at essentially the same pressure. One higher pressure column operates at a lower pressure than the other. This is achieved by maintaining the composition in the bottom of one lower pressure column more oxygen-lean than the other - - the higher pressure column which is thermally linked to the lower pressure column having the more oxygen-depleted composition can thereby operate at lower pressure. Olszewski also teaches to pass oxygen-depleted vapor to the other lower pressure column.
None of the three patents discussed above teaches modes of operation using pumped-LOX.
U.S. Pat. No. 4,433,989 (Erickson) also teaches the use of a third column to improve efficiency. Erickson teaches the use of a third intermediate pressure column in conjunction with a double-column process. The steps taught by Erickson include: 1) passing all the air to the higher pressure column; 2) passing essentially all the oxygen-enriched liquid from the higher pressure column into the intermediate pressure column; 3) distilling in the intermediate pressure column to produce a nitrogen-rich vapor and a further oxygen enriched liquid; 4) passing the further oxygen-enriched liquid to the lower pressure column; 5) refluxing both intermediate pressure column and lower pressure column with nitrogen-enriched liquid from the higher pressure column; and 6) providing boilup to both the intermediate pressure column and the lower pressure column by indirect heat exchange with condensing vapor from the higher pressure column
Erickson also suggests an operating method using pumped-LOX. Erickson teaches that pressurized air is passed to the bottom of a fourth distillation column. This distillation column produces a nitrogen-rich liquid from its top and an oxygen-enriched liquid from its bottom--much like a typical higher pressure column would. The condenser for this fourth column is operated by vaporizing the oxygen product at elevated pressure.
It is desired to have an efficient process for separating air to produce oxygen and nitrogen, wherein the oxygen is produced as a pressurized product and at least a portion of the nitrogen is produced as a pressurized product.
It also is desired to have an efficient mode of utilizing pumped-LOX in a multi-column cycle comprising three or more distillation columns.